Not sure if smps is power supply but that would be my first swap out.
The main board shouldnt be doing this unless its cooked as for it is in a soft power state when power applied to computer and then wen keystroke or power button on computer is used it gets its signal from the soft power state and then boot to the normal operating state.
Causes for a shutdown would be voltage drop out or fan signal not detected as tach pulse. However a system normally will not try to reboot itself this early in the boot process. Its usually an attempted boot and then it shuts down. However a troubled or overloaded power supply is capable of forcing the motherboard to boot and stop and boot and stop over and over again.
If you have already swapped out the power supply with a known good power supply then my thoughts on this are that the main board has an issue with its soft power state when it tries to get it to the next step and drops back to trying again and again and this is likely a cooked board.
Any history of what lead up to this problem such as a bad storm or brown out and then it doesnt want to work, or it was running one day and stopped working in the middle of running to where it may have overheated or?